1. Field of the Invention
The present disclosure relates to random access wireless networks and more specifically to a system and method of modifying a parameter (such as a backoff timing or a mode governing contention capabilities of nodes) in one or more nodes within a wireless network that includes a mixture of full-duplex and half-duplex stations in order to modify a probability that a node or an access point will gain access to the channel to communicate data.
2. Introduction
In wireless networks, such as 802.11 wireless networks, a standard configuration includes a single access point and variety of mobile stations that contend for communication with the access point. In such environments, a number of different coordination functions can manage multi-station access to such an access point. One such function as a distributed coordination function that will cause each station to first check to see that a radio link to the access point is clear before transmitting. To avoid collisions from neighboring stations, each mobile station uses as a random backoff timing after each frame with the first transmitter (i.e., the transmitter having selected the lowest random number) seizing the channel for the radio link. FIG. 2 illustrates the backoff timing via a contention window.
As shown in FIG. 2, the interframe spacing relationships 200 include a busy period 202 in which a station is communicating with the access point, as well as a period of time 204 which can be one of a number of interframe spacings. For example, a “DIFS” is a distributed coordination function interframe space which is defined as a minimal medium idle time for a contention-based service after which stations can have immediate access to the access point if it has been free for a period longer than the DIFS. A “PIFS” is a point coordinator interframe space in which stations with data to transmit in the contention per period can transmit after the PIFS has elapsed and preempt any contention based traffic. An “SIFS” is a short interframe space that is used for the highest priority transmissions such as RTS/CTS frames (requests to send and clear to send).
After such interframe spacing, a contention window 206 is next utilized. The contention window or backoff window follows the interframe spacing and is divided into time slots called “backoff slots.” If multiple stations are seeking to communicate, the stations will pick a random slot and then wait for that slot before attempting to access the access point. All slots are equally likely selections. The station that picks the first slot (or the station with the lowest random number) wins the chance to communicate. All numbers are equally likely to be chosen. In some exceptions, voice-over-IP traffic, which is much more sensitive to poor network service than other data applications, may transmit voice frames with a zero backoff rather than selecting a backoff slot number randomly as is required by some wireless standards. Once a station wins a time slot during the backoff window 206, frame transmission 208 proceeds.
The contention window or the backoff time can be increased to include a larger range of possible timeslots when a transmission fails. In some standards, the contention window sizes are always one less than a power of two, such as 31, 63, 127, 155. If a retry counter increases, the contention window expands to the next greatest power of two. In some cases, the physical layer limits the contention window to a particular number such as 1,023 transmission slots. The window size can then be reset to its minimum size after the frames are transmitted successfully.
The above approach does include drawbacks in that all slot numbers are equally likely to be chosen, fairness can nevertheless be difficult to achieve between the stations for a number of reasons.